Critical Speed Calculator

criticalspeed.gifThe Critical Speed Calculator determines the critical speed of a selected arc or curve. Selecting an arc will generate a single critical speed value. To open the Critical Speed Calculator, select it from the 3D - Recon - Animate section of the left-hand toolbox.

Selecting a curve will generate eight different critical speed solutions. The curved portion will automatically be divided into eight sections and a critical speed value will be calculated for each section. The first two sections are divided into 1/4 and 1/3 the overall length of the curve and are labeled as such. Both sections start at one end of the curve. Six equal and overlapping sections are generated over the entire curve. They’re labeled R1 thru R6. This gives you an opportunity to compare critical speed values at different sections of the curve.

Using the Critical Speed Calculator

You can open the Critical Speed tool with or without pre-selecting objects. If you open the tool without selecting an object or objects then the whole diagram will be displayed in the preview area. If you pre-select an arc or curve before opening the tool then only the selected item or items will be displayed in the preview.

Once an object or objects are displayed in the Critical Speed preview window you can click on an arc or curve to immediately calculate critical speed values.

If you open the Critical Speed tool without first selecting an object or objects in the drawing, the entire drawing is displayed in the preview window. If you select objects first, only those objects will be displayed in the preview.

Clicking on curve in the preview automatically sections and labels the radius measured in the data grid.

Features of the Critical Speed Calculator

All of the data extracted from the selected arc or curve is displayed in a data grid. The data in the blue shaded cells can be modified by the user. This includes Drag and Super Elevation values. There is a User Data row that allows you to manually enter in all the variables to determine critical speed. The User Data section does not use or require arcs or curves to be selected.

crit-speed-data.gif

Drag (u) and Super Elev % (e)

You can change the drag and super elevation values for an individual arc or all of the arcs. Clicking on a cell in the Drag (u) column will bring up a drag value entry box.

Enter the drag value and click OK. You will be prompted to “Change the Drag for all arcs?” If you answer “Yes” then the value you entered will be placed in all of the cells in the Drag (u) column. Answering “No” allows you to enter different drag values for different arcs.

Click on a cell in the Super Elev % (e) column and enter a value (as a percentage) for the super elevation of the road. This would be a slope % of the roads cross section. Like Drag, you will be prompted to “Change the Super Elevation for all arcs?” If you answer “Yes” then the value you entered will be placed in all of the cells in the Super Elev % (e) column. Answering “No” allows you to enter different super elevation values for different arcs.

Flip Super Elevation and Flip R Order

Checking the “Flip Super Elevation” box will change the super elevation values from positive to negative and visa versa. This changes the direction of the slope.

Checking the “Flip R Order” box will change the end of the curve from which the arc sections start.

NOTE: This will not change the values in the Drag (u) and Super Elev % (e) columns.

Display Calculations: When checked, the critical speed calculations are displayed on the screen along with the selected arc or curve and can be placed in the diagram.

Export (Save): Use this button to save and export the critical speed. The data is saved in a .Txt, .CSV, .DAT, or .ASC format.

Place: Closes the Critical Speed Calculator and places the diagram shown in the Preview Window in the drawing screen.

If the drawing screen already contains a diagram, the drawing appears attached to the mouse pointer allowing you to position and place it where you want it in the existing drawing.

Close: Closes the Critical Speed Calculator.

Formulas Used in the Critical Speed Calculator

u = coefficient of friction

e = Super Elevation (%)

C = Chord

m = middle ordinate

r = Radius

S = Speed

r = C^2/8m + m/2

The Critical Speed Calculator uses two different formulas to make the calculations, based on the Super Elevation value.

Super Elevation  <= 10% Formula : S = (3.86 * sqrt(r*(u+e)))

Super Elevation  > 10% Formula : S = (3.86 * sqrt(r*(u+e))) / sqrt(1-u*e)

See Also:

Skid Analysis, Momentum Angles, Momentum Calculator, Crush Calculator, Reconstruction Tutorial

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